Breaking of Water-in-Crude Oil Emulsions. 1. Physicochemical

1600

Energy & Fuels 2006, 20, 1600-1604

Breaking of Water-in-Crude Oil Emulsions. 1. Physicochemical Phenomenology of Demulsifier Action Miguel Rondo´n, Patrick Bouriat, and Jean Lachaise Laboratoire des Fluides Complexes, UMR 5150, UniVersite´ de Pau et Pays de l’Adour, France

Jean-Louis Salager* Laboratorio FIRP, UniVersidad de Los Andes, Me´ rida, Venezuela

Downloaded via UNIV OF WINNIPEG on January 23, 2019 at 14:15:34 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

ReceiVed January 12, 2006. ReVised Manuscript ReceiVed April 12, 2006

Water-in-oil emulsions formed during oil slicks or petroleum production are known to be stabilized by surfactant molecules that naturally occur in the crude oil, e.g., asphaltenes, which are quite lipophilic in nature. Demulsifier substances combine with naturally occurring surfactants to attain a so-called optimum formulation at which the stability of the emulsion is minimum. The attainment of this formulation is related to the hydrophilicity and concentration of the added demulsifier, and a general phenomenology of the demulsification process is outlined.

Introduction Petroleum is most often produced as a water-in-oil emulsion and the water must be removed (down to a level of 12) indicates a lipophilic (or hydrophilic) surfactant and, hence, results in a W/O (or O/W) emulsion, according to Bancroft’s rule. Original studies on the HLB effect on emulsion properties reported the presence of a stability maximum at some value, the so-called required HLB, which is dependent on the nature of the oil; however, there was no systematic screening until the investigation by Boyd et al.,6 which indicated that there are two stability maxima (one at HLB < 10 (W/O emulsion) and one above it (O/W)), and, therefore, there is an implicit stability minimum between them, at HLB ≈ 10. The HLB scale was empirical and had several shortcomings, particularly the fact that it was inaccurate when comparing different surfactant families and it did not take into account the effects of temperature, salinity, and the nature of the hydrophilic group. It may be said that the HLB scale, which is still used today, because of its extreme simplicity, is only valid to compare substances in a same family of surfactants. The enhanced oil recovery research drive in the 1970s allowed researchers to improve upon Winsor’s R-ratio7 and Shinoda’s phase inversion temperature (PIT) concepts8 to describe the (4) Griffin, W. C. J. Soc. Cosmet. Chem. 1949, 1, 311-326. (5) Griffin, W. C. J. Soc. Cosmet. Chem. 1954, 5, 249-256. (6) Boyd, J.; Parkinson, C.; Sherman, P. J. Colloid Interface Sci. 1972, 41, 359-370. (7) Winsor, P. SolVent Properties of Amphiphilic Compounds; Butterworth: London, 1954. (8) Shinoda, K.; Arai, H. J. Phys. Chem. 1964, 68, 3485-3490.

10.1021/ef060017o CCC: $33.50 © 2006 American Chemical Society Published on Web 05/18/2006

Breaking of Water-in-Crude Oil Emulsions. 1

Energy & Fuels, Vol. 20, No. 4, 2006 1601

Figure 1. (Left) Stability as the time required to attain 50% of separated water from different emulsions (see Table 1), as a function of three different formulation variables (HLB, EON, and salinity). (Right) Stability of the same emulsions as a function of HLD. Table 1. Compositions of the Different Emulsions Used in Figure 1 symbol

surfactant

oil

salinity

alcohol

squares, 0 circles, O triangles, 4

SDS (10000 ppm) nonylphenols (10000 ppm of 3EO and 6EO) Tween 20 and Span 80 (250 ppm)

kerosene lubricating oil cyclohexane

variable 0% 0%

pentanol (47600 ppm) sec-butanol (20000 ppm) none

physicochemical formulation. The quantitative rendering of all effects through the so-called surfactant affinity difference (SAD)9 or its dimensionless equivalent expression (i.e., the hydrophilic-lipophilic deviation, HLD10) was essentially no more that an accurate numerical equivalent of the HLB concept that takes into account not only the surfactant but also other formulation variables such as oil nature, aqueous phase salinity nature and concentration, and the presence of alcohol cosurfactant, as well as temperature and pressure. A typical expression of HLD for a polyethoxylated nonionic surfactant may be written as

HLD ) kβ - kACN + bS + mCA + cT(T - 25 °C) (1) where β is a characteristic parameter of the surfactant, ACN is the alkane carbon number (and is characteristic of the alkane oil used), S is the salinity (wt % NaCl), CA is the concentration of alcohol cosurfactant is any, and T the temperature. The parameters k, b, m, and cT are constants whose values are dependent on the nature of the components. Note that the surfactant parameter kβ, which is often written as [R-EON], is dependent on both the headgroup (as the number of ethylene oxide units per molecule, EON) and the tail length (R). Whenever HLD is positive or negative, the emulsion type is O/W or W/O, provided that the water-oil ratio is relatively close to unity. Other cases, when the water-oil ratio departs from unity, are dealt with elsewhere.11 Hence, HLD is a generalized formulation concept that is essentially equivalent to Bancroft’s rule, HLB and PIT, with the advantage of numerically expressing the contribution of each formulation variable, so that compensations and counter-effects can be handled in a predictive way. Systematic studies on emulsion stability have indicated that there is a strong correlation between the generalized formulation (HLD) value and the overall phenomenology indicated in Figure 1, regardless of the variable that produces the change in formulation, e.g., salinity of the aqueous phase, surfactant hydrophilicity, or other.12-16 Table 1 gives the compositions of the emulsions used in Figure 1. The minimum stability occurs at HLD ) 0, which is the socalled optimum formulation at which the equilibrated system exhibits, in most cases, an ultralow minimum tension, eventually

a three-phase behavior, and a minimum viscosity, whose combination produces the appropriate situation for enhanced oil recovery by surfactant flooding.17 Note that, because optimum formulation is associated with low tension, the produced drop size is probably lower in this region, hence the settling is expected to be slower, with a resulting higher stability. The experimental evidence just shows the opposite. Therefore, there is no doubt that formulation is the main influence on stability in this region. Various explanations18-22 have been proposed for the occurrence of a very deep stability minimum at optimum formulations and, although they are quite different, they all point to the same phenomenology. Close to optimum formulation, drops essentially coalesce upon contact, i.e., the delay for phase separation is related to the settling velocity and everything happens as if there was no surfactant at the interface.19 The position and intensity of the stability maxima that occur on both sides of HLD ) 0 are dependent on secondary effects as the strength of the electrical or steric repulsions, eventually produced by the adsorbed or deposited amphiphiles.2 For instance, asphaltenes are known to form thick multiple (9) Salager, J. L. Pharmaceutical Emulsions and Suspensions; Nielloud, F., Marti-Mestres, G., Eds.; Marcel Dekker: New York, 2000; Chapter 2, pp 19-72. (10) Salager, J. L.; Ma´rquez, N.; Graciaa, A.; Lachaise, J. Langmuir 2000, 16, 5534-5539. (11) Salager, J. L. Pharmaceutical Emulsions and Suspensions; Nielloud, F., Marti-Mestres, G., Eds.; Marcel Dekker: New York, 2000; Chapter 3, pp 73-125. (12) Bourrel, M.; Graciaa, A.; Schechter, R. S.; Wade, W. H J. Colloid Interface Sci. 1979, 72, 161-163. (13) Salager, J. L.; Quintero, L.; Ramos, E.; Ande´rez, J. M. J. Colloid Interface Sci., 1980, 77, 288-289. (14) Salager, J. L.; Loaiza-Maldonado, I.; Min˜ana-Pe´rez, M.; Silva, F. J. Dispersion Sci. Technol. 1982, 3, 279-292. (15) Milos, F. S.; Wasan, D. T. Colloids Surfaces 1982, 4, 91-99. (16) Kim, Y. H.; Wasan, D. T. Ind. Eng. Chem. Res. 1996, 35, 11411149. (17) Shah, D. O., Schechter, R. S., Eds. ImproVed Oil RecoVery by Surfactant and Polymer Flooding; Academic Press: New York, 1977. (18) Langmuir I. J. Am. Chem. Soc. 1917, 39, 1848-1906. (19) Anto´n, R. E.; Salager, J. L. J. Colloid Interface Sci. 1986, 111, 54-59. (20) Hazlett, R. D.; Schechter, R. S. Colloids Surfaces 1988, 29, 5369. (21) Kabalnov, A.; Wennerstro¨m, H. Langmuir 1996, 12, 276-292. (22) Kabalnov, A.; Weers, J. Langmuir 1996, 12, 1931-1937.

1602 Energy & Fuels, Vol. 20, No. 4, 2006

Rondon et al.

layers.23 Some surfactant types are more efficient than others to increase stability, but their concentration is also important. For instance, in Figure 1, the stability curve at 250 ppm of Span-Tween mixtures exhibits much less stability in offoptimum formulations essentially because the surfactant concentration is too low, much lower than the typically 3-5000 ppm required to attain a stable emulsion.24 2. Principle of Demulsifier Action. Optimum formulation at HLD ) 0 is the formulation condition that is associated with minimal emulsion stability and, hence, is the desired situation for emulsion breaking.25 Naturally occurring W/O emulsions, which are stabilized by lipophilic “natural surfactants”, are located on the HLD > 0 side and the demulsification essentially consists of adding a second surfactant to shift the formulation to HLD ) 0.26,27 Consequently, the demulsifier is a hydrophilic surfactant, and its contribution is such that its mixture with the natural surfactants produces an interfacial mixture which is exactly at HLD ) 0. This contribution may be expressed as a mixing rule on the surfactant parameter28 that results from the mixture of the natural surfactant and demulsifier species. For the sake of simplicity, the old HLB scale may be used for reasoning, particularly because it is easy to relate with the ethoxylation degree of nonionic surfactants, which are used as demulsifiers in the experimental part. If the system contains several surfactants labeled i ) 1, 2, 3, ..., the HLB of the blend, HLBm, is a function of the hydrophilicities of the different components (HLBi) and their proportions in the interfacial mixture (Xi). The simplest case is observed when the mixing rule can be expressed as a linear relationship, such as

HLBm )

∑i XiHLBi

(2)

At interfaces, systems that are under study can be assimilated to a binary mixture of an asphaltenic surfactant pseudocomponent (A) and a demulsifying substance pseudo-component (D). Equation 2 then can be written as

HLBm ) XAHLBA + XDHLBD ) (1 - XD)HLBA + XDHLBD

(3)

At a given interfacial composition, corresponding to the minimum stability of emulsion, the interfacial pseudo-component mixture is such that HLD ) 0. In the case where T ) 25 °C, the salinity and alcohol content are zero, and the nature of the oil (ACN) is constant, eq 1 indicates that HLD is just a function of the nature of interfacial surfactant mixture. One may conclude that, in this case, whatever the reason that leads to HLD ) 0, the HLB of the optimal mixture is constant and equals HLB*m. For instance, the lefthand side of Figure 1 (triangle symbols) shows that HLB* m ≈ 10 when the oil phase is cyclohexane. (23) McLean, J. D.; Kilpatrick, P. J. Colloid Interface Sci. 1997, 189, 242-253. (24) Ande´rez, J. M.; Bracho, C. L.; Sereno, S.; Salager, J. L. Colloids Surf. A 1993, 76, 249-256. (25) Salager, J. L. Int. Chem. Eng. 1990, 30, 103-116. (26) Krawczyk, M. A.; Wasan, D. T.; Shetty, C. S. Ind. Eng. Chem. Res. 1991, 30, 367-375. (27) Goldszal, A.; Bourrel, M. Ind. Eng. Chem. Res. 2000, 39, 27462751. (28) Salager, J. L.; Bourrel, M.; Schechter, R. S.; Wade, W. H. Soc. Pet. Eng. J. 1979, 19, 271-278.

Table 2. HLB and EON Values of the Polyethoxylated Nonylphenol Surfactants Used EON

HLBD

4.75 5.5 10 15 20

9.7 10.4 13.3 14.9 16.6

The HLB value that gives the fastest separation for a given interfacial proportion XD of demulsifier may be expressed in terms of HLB* m and HLBA as

HLB*D )

HLB*m - (1 - XD)HLBA XD

(4)

This is the HLB value of the optimum demulsifier for a given concentration that corresponds to XD, which is an interfacial proportion. It is important to note that the optimum demulsifier is not optimum for all concentrations. Similarly, if a demulsifier with HLBD is selected, there is a certain optimum concentration of it that yields the fastest separation. The corresponding optimum fraction at the interface is given by

X*D )

HLB*m - HLBA HLBD - HLBA

(5)

The asphaltenes and other amphiphilic substances that adsorb at the interface from the crude oil are quite lipophilic (HLB < 10) as they form W/O emulsions. As X*D < 1, eq 5 implies that HLBD > HLB*m > HLBA, and then essentially that HLBD > 10. X* D or HLB* D cannot be calculated from eqs 4 and 5, because the HLB*m, HLBA, and HLBD values are not all known. Consequently, the optimum physicochemical situation must be determined experimentally, using a hydrophilic demulsifier, or a family of hydrophilic demulsifiers, to offset the influence of the intrinsic lipophilic character of the asphaltene surfactant pseudo-component. Experimental Section 1. Water and Oil Phases. Distilled water or different surfactants aqueous solutions at different concentrations are used as the aqueous phase. A French crude oil (Vic-Bilh Oil, extracted by Total in France) containing 10 wt % asphaltenes was diluted 100 times in cyclohexane to comprise the oil phase. Hence, unless otherwise stated, the oil phase may be essentially considered to contain 1000 ppm of asphaltenes in cyclohexane. Diluted crude oil was used to reduce the viscosity of the organic phase and obtain a faster water separation. Cyclohexane was chosen because previous studies have shown that water droplets do not coalesce in this oil mixture, even if the concentration of asphaltenes is decreased. 2. Demulsifier Surfactants. In the main study, polyethoxylated nonylphenol nonionic surfactants with different average degrees of ethoxylation (referenced as EON) are used as demulsifiers. They are blends of polyethoxylated nonylphenol that have been provided by SEPPIC, France. These substances are known to be able to produce a demulsifying effect; they may not be as efficient as the actually used products, but they have a smaller molecular weight and are much better-defined; consequently, it is easier to perform accurate formulation scans by mixing them. Moreover, their fractionation behavior is well-known and their interfacial mixture composition may be estimated.29 HLB values of these surfactants are given in Table 2. (29) Graciaa, A.; Lachaise, J.; Sayous, J. G.; Grenier, P.; Yiv, S.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1983, 93, 474486.

Breaking of Water-in-Crude Oil Emulsions. 1

Energy & Fuels, Vol. 20, No. 4, 2006 1603 Table 3. Optimal Concentrations of Different Ethoxylated Nonylphenols Used as Demulsifiers

Figure 2. Bidimensional map of emulsion stability versus the nature and concentration of demulsifier.

A secondary study is conducted with Pluronic PE 6200 and PE 6400, furnished by BASF. These poly(ethylene oxide)-polypropyleneoxide-poly(ethylene oxide) triblock nonionics are not so well defined but are actually used as demulsifiers. HLB of these products were determined experimentally by blending them in variable proportions with an emulsifier of known HLB, to find again the optimum formulation previously determined with a mixture of Tween and Span. Using this procedure, we found that the HLB of PE 6200 (or PE 6400) is ∼10.5 (or ∼15). When the desired HLB value needed for an experimental scan was not available, it was obtained by blending two surfactants that had the nearest HLB values, on a weight fraction basis. 3. Measurement of Emulsion Stability (Bottle Tests). Bottle tests are performed with 10-mL total volume samples at a unit water-oil ratio. Demulsifier surfactants are added in the water phase at the corresponding concentration. The water and oil phase then are left to equilibrate 24 h at ambient temperature. Emulsification is then conducted in a plastic conical beaker with an Ultraturrax turbine blender at 1800 rpm during 30 s. The pre-equilibration process before emulsification warranties that the general phenomenology between formulation and emulsion type and properties14,30 applies. If the SOW system were not pre-equilibrated, transient mass-transfer effects could alter the results and hinder the interpretation, as recently discussed elsewhere.31,32 The emulsion persistence is appraised by monitoring the separated water as a function of time. The time (in minutes) elapsed for half the total volume of water to separate is taken as a measurement of the emulsion stability for each system, for different demulsifier types (EON) and concentrations.

Results and Discussion 1. Optimum Formulations. The stability of emulsions prepared with a certain amount of asphaltene was determined for different demulsifier formulations. The HLBD value of the demulsifier (polyethoxylated nonylphenol series) was scanned at constant demulsifier concentration, hence, constant XD (XD ) 1 - XA), with an ethoxylation degree ranging from EON ) 4.75 to EON ) 20, i.e., from HLBD ) 9.7 to HLBD ) 16.6. For each HLBD scan at constant demulsifier concentration, a stability minimum is observed, as shown in the left plot of Figure 2, which is essentially similar to the behavior exhibited in Figure 1. On the other hand, demulsifier concentration (hence, XD) scans are conducted at constant demulsifier HLBD. Such plots also exhibit an emulsion stability minimum at a given concentration, as illustrated in the graph on the right-hand side in Figure 2. (30) Salager, J. L.; Min˜ana-Pe´rez, M.; Pe´rez-Sa´nchez, M.; Ramı´rezGouveia, M.; Rojas, C. I. J. Dispersion Sci. Technol. 1983, 4, 313-329. (31) Salager, J. L.; Moreno, N.; Anto´n, R. E.; Marfisi, S. Langmuir 2002, 18, 607-611. (32) Alvarez, G.; Anto´n, R. E.; Marfisi, S.; Marquez, L.; Salager, J. L. Langmuir 2004, 20, 5179-5181.

HLBD

CD* (ppm)

9.7 10.4 13.3 14.9 16.6

400 200 50 30 20

These two stability minima correspond to two different ways to attain the optimum contribution of the demulsifier, XDHLBD, either by optimizing HLBD at constant XD or vice versa. This double scan allows us to build a bidimensional map with isostability contours, as indicated in Figure 3. The map on the left-hand side in Figure 3 was constructed with ethoxylated nonylphenol mixtures, which allow a careful scanning of the HLBD, with accurate and continuous variation. The isostability contours are slanted lines that are roughly parallels. The stability value is increasing as the contours depart from the stability minimum (white) region, which surrounds a minimum stability occurrence (indicated as a dashed bold line). The experimental values of these stability minima are reported in Table 3. Note that, from the point of view of the demulsification process, the white region corresponds to a separation time that is short enough to be feasible for practical purposes. The map on the right-hand side in Figure 3 was made using Pluronic 6200 and 6400, which are two poly(ethylene oxide)polypropylene oxide (PEO-PPO) triblock surfactants that are actually used in practice for demulsifying purposes. Although there are only two different surfactants to perform the scan, the stability pattern is essentially the same as that observed with the polyethoxylated nonylphenols in the same HLB scale range. This corroborates that ethoxylated nonylphenol surfactants could be used for accurate formulation studies. In both cases, the slanting of the dashed bold line indicates that there is a tradeoff between the demulsifier hydrophilicity (HLBD) and its amount (XD) to reach an optimum formulation. The tradeoff is probably not accountable, because the interfacial values are unknown; however, it probably follows such a trend, i.e., when demulsifier HLBD is higher, its interfacial concentration XD is lower, and vice versa. 2. Optimum Optimorum (i.e., the Best of All Optima). However, all optimal formulations do not have the same effect on the emulsion stability. Figure 4 shows the stability variation, as a function of demulsifier concentration. For each demulsifier (labeled by its EON), the minimum is indicated as a big black circle. The locus of minima (i.e., the envelope of the stability curves) undergoes a minimum minimorum (i.e., the lowest of all minima), indicated in Figure 4 as a star-shaped symbol (f) at a demulsifier concentration of ∼100 ppm and an EON value close to 8. This optimum optimorum situation is the best compromise between a large amount of a slightly hydrophilic demulsifier (EON ) 4.75) and a small amount of a very hydrophilic demulsifier (EON ) 20). A very hydrophilic surfactant would have a tendency to remain in water with a very small driving force to adsorb at the interface, particularly because it is present at a low concentration. Moreover, its higher molecular weight would result in a slower kinetics. On the other hand, a slightly hydrophilic mixture would contain lipophilic species likely to partition in the oil phase, as reported elsewhere.29 The resulting change in formulation at the interface, i.e., an increase in molecular weight, would also slow the kinetic effects. This is consistent with previous works, which have indicated that the presence of smaller molecules has a tendency to increase the

1604 Energy & Fuels, Vol. 20, No. 4, 2006

Rondon et al.

Figure 3. Isostability contours as a function of demulsifier type and concentration for systems containing 1000 ppm of asphaltenes in cyclohexane: (left) ethoxylated nonylphenols, and (right) PEO-PPO-PEO block polymers.

not so wide: Any error in dosification would produce an increase in stability. In contrast, the stability curve for the EON ) 10 demulsifier is quite flat in the vicinity of the minimum, and this feature provides a large margin for error in dosification. The choice should also take into account secondary features such as the aspect of the water after demulsification, which would have a tendency to become turbid at HLD < 0, because the micelles are able to solubilize some oil. In the same order of ideas, an HLD value close to 0 or slightly positive would produce a not-so-well water-wet system, and intermediately wet solid particles could enhance their stabilization effect, at least close to optimum formulation. Conclusion

Figure 4. Stability (as time for 50% of water separation) versus demulsifier concentration at constant surfactant EON for different EON values over the 4.75-20 EO range. The minimum stability for each EON is indicated as a black circle. Minimum of the envelope (optimum optimorum) is indicated by a star (f) symbol.

mass-transfer velocity and the coalescence mechanism close to optimum formulation.32,33 Note that there is often some margin to adjust a demulsifier formulation. For instance, Figure 4 shows that the EON ) 10 demulsifier is essentially as efficient as the optimum demulsifier (EON ) 8), but it requires less demulsifier and is, thus, probably a more-practical choice. The same could be said for the EON ) 5.5 demulsifier; however, its stability curve is not so flattened and the range of concentration associated with low stability is (33) Lopez, E. Influence of formulation on the stability of emulsions (in Sp.). MSc. Thesis in Chemical Engineering, University of the Andes, Me´rida Venezuela, 2004.

Demulsifier action essentially consists of the combination at the interface of the added species with asphaltenes until a socalled optimum formulation is attained, at which the interfacial amphiphile mixture exhibits the same affinity for both phases. The contribution of the demulsifier includes both its intrinsic hydrophilicity and its concentration. The more hydrophilic it is, the lesser the required amount to attain the proper formulation at the interface. However, a best-of-all condition for breaking emulsion is observed when there is some compromise, i.e., when using a not-too-hydrophilic demulsifier in a not-too-small concentration. Acknowledgment. The authors thank the French Ministry of Research for the financial support (Nr 04 G 366) within the framework of RITMER. They also acknowledge the financial backing provided by FONACIT-Venezuela (Grant S1-2001-001156, PCP program on “Petroleum Emulsions”) and University of the Andes Research Council (Grant No. I-834-05-08-AA). EF060017O